Recently, Internet Protocol (IP) network call service utilizing an IP increasingly spreads. Accordingly a technique to switch where to connect a telephone selectively between an IP network and a public network is indispensable. (This technique will hereinafter also be referred to as “the switching technique”.) A public network, as referred to herein, indicates public switched telephone networks (PSTN).
Such techniques are disclosed in Japanese Patent Laying-open Nos. 2004-007314 (Patent Document 1), 2005-005875 (Patent Document 2), and 2005-086286 (Patent Document 3).
Japanese Patent Laying-open No. 2004-007314 (Patent Document 1) discloses that a telephone rate table previously prepared as based on telephone numbers is used to switch where to connect a telephone selectively between an IP network and a public network.
Japanese Patent Laying-open No. 2005-005875 (Patent Document 2) discloses connecting a telephone, as switched in response to a calling signal of a telephone network or an IP network, and connecting the telephone to the telephone network when a power failure occurs.
Japanese Patent Laying-open No. 2005-086286 (Patent Document 3) discloses that when a power failure is detected, the power of a public network is used to dial from a telephone network to the person that had been calling before the power failure to resume the call.    Patent Document 1: Japanese Patent Laying-open No. 2004-007314    Patent Document 2: Japanese Patent Laying-open No. 2005-005875    Patent Document 3: Japanese Patent Laying-open No. 2005-086286